A vehicle dynamic control system, such as, for example, an active steering system, an active braking system and/or an active traction control system, may be responsive to a time-wise rate of change in the yaw angle (xe2x80x9cyaw ratexe2x80x9d), generally expressed in radians per second. In an active steering system, for example, the yaw rate may be monitored in order to generate a steering angle output that is additive to an operator commanded steering angle during severe maneuvers. In order to correct transient steering instabilities, such an active steering system may control a vehicle more quickly and accurately than an average driver.
Since 1996 brake suppliers have offered vehicle stability systems which use differential braking to improve the handling feel for vehicles operating on slick surfaces. These systems typically employ a yaw rate sensor to detect unwanted vehicle slew from the driver""s intended path and apply pressure to the brakes on one wheel to bring the vehicle back to the desired attitude. For example, in over-steer conditions, where the rear end of the vehicle starts to spin out, loss of tire adhesion to the roadway prevents the rear tires from exerting a restoring force to return the vehicle to the desired attitude. Thus for optimum vehicle stability, a brake stability system is recommended to provide differential braking to the front wheels (which still retain good roadway adhesion) for control of the over-steer spinout. Failure of a yaw sensor could provide an unwanted brake drag in this control system. Accordingly, these brake systems must provide a sensitive diagnostic to disable the stability function when the yaw rate sensor has failed.
A four-wheel active steering system may steer the front and the rear wheels of a vehicle. More specifically, a rear wheel steering portion of the system may produce a desired rear wheel steering angle to achieve tight turning radii at low speeds by steering the front and rear wheels in different directions to thereby reduce the effective turning radius of the vehicle. At generally higher speeds, the four-wheel active steering system may enhance cornering stability by steering the front and the rear wheels in relatively similar directions.
Many motor vehicle handling instabilities are generally manifested as a function of the yaw rate of the vehicle. For example, the yaw rate may be influenced by transient changes in the relative slip angles of the tires while the vehicle is negotiating a corner at or near the situation-specific limits of adhesion in the presence of irregular road surface conditions. An increase in slip angle at one end of the vehicle generally leads to more lateral movement of that end relative to the other end to thereby affect the yaw rate.
The use of rear wheel steering to provide vehicle yaw stability allows very quick system response and provides a very smooth transition to a new desired vehicle track when changing lanes on slick road surfaces. Under most conditions, it does not require the brakes to activate during the transition. This eliminates the normal deceleration tugs required by the braking system to correct undesired veer in the lane change maneuver. Unfortunately, the quick response provided by the steer controller places greater demands upon the detection logic used for verification that the yaw rate sensor in the system is functioning properly. For the slower response of the brake stability system a fault detection process that allows several seconds to respond to a sensor failure provides adequate assurance that the vehicle driver is not subjected to a stressful operating condition. In a steer stability system a maximum yaw sensor error must be diagnosed in less than a second to prevent an undesired veering into an adjacent roadway lane. Thus, there is a desire to enhance yaw rate estimation for comparison with yaw rate sensor measurements to quickly detect sensor errors. The true challenge for any automotive stability control system is the ability to rapidly identify a system failure. The system must respond quickly to changing environments, which could lead to loss of control, but at the same time must detect any sensor failure that might mimic a loss-of-control situation.
The above described and other features are exemplified by the following Figures and Description in which a vehicular system is disclosed that controls the method for estimating the yaw rate using other sensors for correlation with an actual measured yaw rate from a yaw rate sensor. The method includes receiving at least one signal indicative of a vehicular lateral acceleration and receiving at least one signal indicative of a vehicular wheel velocity. A plurality of yaw rate estimation functions are provided. A first yaw rate estimation function of the plurality of yaw rate estimation functions is selected in response to at least one of the received signals. A first estimated yaw rate is estimated in accordance with the selected first yaw rate estimation function and at least two signals each indicative of a wheel velocity. If the first estimated yaw rate is not within a threshold value of an actual measured yaw rate, a second yaw rate estimation function is selected to obtain a second estimated yaw rate using a signal indicative of lateral acceleration for correlation with the actual measured yaw rate. If the second estimated yaw rate is not within the threshold value, a signal indicative of a yaw rate sensor fault is generated.